A. Field of the Invention
This invention is in the field of liquid storage tanks with a double deck floating roof, and particularly a liquid petroleum storage tank with a floating roof and a drainage system to drain excess accumulation of water on top of the floating roof.
B. Background of the Invention
Storage tanks for oil and other liquid petroleum products are typically provided with a floating roof that covers and protects the liquid product, prevents the escape of harmful vapors, and moves vertically on the surface of the liquid product as its volume changes. A typical floating roof for covering a liquid product in a storage tank includes a deck that substantially conforms to the horizontal cross-sectional shape of the storage tank and has a vertical thickness. The roof is provided with one or more seals extending between the outer periphery of the floating roof and the inner walls of the tank. The roof floats on top of the stored liquid product and rises and falls as the amount of the liquid product increases and decreases. It is known to provide a drain pipe extending vertically through the top surface of deck to drain water accumulated on the top of the roof and to provide a passage or conduit for the water. The water drained from the surface of the roof passes through the stored liquid to a discharge outlet at the bottom of the tank where it is eventually drained from the tank.
During seasonal or other periods of heavy rains the amount of water that accumulates over the area of the tank cover can be substantially greater than that which can be removed by a centrally positioned basic drain. The water will therefore rise and can inundate the drain sealing mechanism. The presence of debris carried by the accumulating water can interfere with the proper operation of complex roof drain apparatus known to the prior art.
A further problem can occur when rainwater accumulates on the roof and its effective weight increases to a point where it can overcome the buoyancy of the roof, eventually forcing it into the liquid in the tank. It is therefore important to provide an effective and reliable means for draining water from the roof, so that it does not exceed a pre-determined depth and weight.
Various approaches have been proposed for draining water from storage tank floating roofs. For example, U.S. Pat. No. 2,560,586 to Michaels discloses a floating roof drain which drains water collected over a valve of the floating roof which closes and opens a drain passing. The weight of the water collected over the valve pushes down a cover against the buoyancy force of a float connected to the valve to allow the water collected to flow from the roof. The valve is again closed when the depth of the water on the roof decreases until it is no longer sufficient to hold the valve open against the buoyancy force of the float.
U.S. Pat. No. 2,913,138 to Swick describes floating covers for tanks in which a drainage device is located at a low point of the roof structure in a deck. The drainage device comprises a cylindrical sump, a sump bottom formed with a shallow depression which constitutes a downward flow passage, a cylindrical neck extending from the bottom of the sump and an annular float member loosely surrounding the sidewall of the neck. A mercury seal is provided on the bottom and the float member rests on the bottom of the pool of mercury. Accumulated water in the sump buoys up the float member to interrupt the mercury seal and thereby provide a conduit for water to drain across the surface of the pool.
U.S. Pat. No. 3,883,032 to Fisher discloses an automatic drain valve for a floating roof which includes an orifice and a larger disk located under the orifice. A float attached to the disk allows the valve to be biased closed, and water gathered on top of the roof will open the valve which permits the water to drain through the roof into the tank beneath.
Japanese Patent No. JP5077883 to Kunio discloses an emergency drain device for a floating roof in which a drain pipe runs through a deck to stored liquid in a tank. A float element adjacent the upper end of the drain pipe floats on accumulated liquid on the deck. A weight-attached guide member makes the float return to the upper end of the drain pipe to prevent vapor from flowing out of the drain pipe.
The above systems for draining water from floating roofs do not provide desired reliability, reduction of hazardous conditions resulting from accumulated hydrocarbon vapors, secured sealing against evaporation losses, and performance under extreme conditions such as high winds and a significant accumulation of rainwater.
It is therefore, an object of the present invention to provide an apparatus for effectively and reliably draining accumulated rain water from a floating roof on a liquid product tank by a self-opening and closing mechanism for a floating roof drain.
An additional object is to provide for a floating roof an emergency drainage system which will automatically open in a predetermined condition.
A still further object is to provide an uncomplicated but reliable emergency drainage system which will be normally closed from a compression spring and will automatically open merely from the accumulation of a predetermined head of water on the roof in the vicinity of the emerging drainage valve.
Another object of the invention is to provide an apparatus for draining accumulated rain water from the top of a roof on a liquid product storage tank and reduction of the hazardous condition of accumulated hydrocarbon vapors and their evaporation to the atmosphere.
A further object of the invention is to provide an automatic roof drain valve that is of simple and rugged construction and inexpensive to manufacture and install, such valve being made of a durable material such as carbon graphite that is corrosion and chemical resistant.